.\" $OpenBSD: CMAC_Init.3,v 1.6 2024/03/02 09:30:21 tb Exp $
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.\" Copyright (c) 2020 Ingo Schwarze <schwarze@openbsd.org>
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.Dd $Mdocdate: March 2 2024 $
.Dt CMAC_INIT 3
.Os
.Sh NAME
.Nm CMAC_CTX_new ,
.Nm CMAC_Init ,
.Nm CMAC_Update ,
.Nm CMAC_Final ,
.Nm CMAC_CTX_copy ,
.Nm CMAC_CTX_get0_cipher_ctx ,
.Nm CMAC_CTX_cleanup ,
.Nm CMAC_CTX_free
.Nd Cipher-based message authentication code
.Sh SYNOPSIS
.In openssl/cmac.h
.Ft CMAC_CTX *
.Fn CMAC_CTX_new void
.Ft int
.Fo CMAC_Init
.Fa "CMAC_CTX *ctx"
.Fa "const void *key"
.Fa "size_t key_len"
.Fa "const EVP_CIPHER *cipher"
.Fa "ENGINE *engine"
.Fc
.Ft int
.Fo CMAC_Update
.Fa "CMAC_CTX *ctx"
.Fa "const void *in_data"
.Fa "size_t in_len"
.Fc
.Ft int
.Fo CMAC_Final
.Fa "CMAC_CTX *ctx"
.Fa "unsigned char *out_mac"
.Fa "size_t *out_len"
.Fc
.Ft EVP_CIPHER_CTX *
.Fn CMAC_CTX_get0_cipher_ctx "CMAC_CTX *ctx"
.Ft void
.Fn CMAC_CTX_cleanup "CMAC_CTX *ctx"
.Ft void
.Fn CMAC_CTX_free "CMAC_CTX *ctx"
.Sh DESCRIPTION
CMAC is a message authentication code algorithm that can employ an
arbitrary block cipher using a symmetric key.
.Pp
The present manual page describes low-level functions implementing CMAC.
Instead of using these functions directly,
application programs normally call
.Xr EVP_PKEY_CTX_new_id 3
with an argument of
.Dv EVP_PKEY_CMAC
and then pass the resulting
.Vt EVP_MD_CTX
object to
.Xr EVP_DigestInit_ex 3 .
.Pp
The CMAC API is object-oriented.
Calculating a message authentication code requires a
.Vt CMAC_CTX
object.
Usually, the functions
.Fn CMAC_CTX_new ,
.Fn CMAC_Init ,
.Fn CMAC_Update ,
.Fn CMAC_Final ,
and
.Fn CMAC_CTX_free
need to be called in this order.
.Pp
.Fn CMAC_CTX_new
allocates a new
.Vt CMAC_CTX
object, initializes the embedded
.Vt EVP_CIPHER_CTX
object, and marks the object itself as uninitialized.
.Pp
.Fn CMAC_Init
selects the given block
.Fa cipher
for use by
.Fa ctx .
Functions to obtain suitable
.Vt EVP_CIPHER
objects are listed in the CIPHER LISTING section of the
.Xr EVP_Cipher 3
manual page.
Unless
.Fa key
is
.Dv NULL ,
.Fn CMAC_Init
also initializes
.Fa ctx
for use with the given symmetric
.Fa key
that is
.Fa key_len
bytes long.
In particular, it calculates and internally stores the two subkeys
and initializes
.Fa ctx
for subsequently feeding in data with
.Fn CMAC_Update .
The
.Fa engine
argument is ignored; passing
.Dv NULL
is recommended.
.Pp
If
.Fa ctx
is already initialized,
.Fn CMAC_Init
can be called again with
.Fa key
and
.Fa cipher
both set to
.Dv NULL
and
.Fa key_len
set to 0.
In that case, any data already processed is discarded and
.Fa ctx
is re-initialized to start reading data anew.
.Pp
.Fn CMAC_Update
processes
.Fa in_len
bytes of input data pointed to by
.Fa in_data .
Depending on the number of input bytes already cached in
.Fa ctx ,
on
.Fa in_len ,
and on the block size, this may encrypt zero or more blocks.
Unless
.Fa in_len
is zero, this function leaves at least one byte and at most one
block of input cached but unprocessed inside the
.Fa ctx
object.
.Fn CMAC_Update
can be called multiple times
to concatenate several chunks of input data of varying sizes.
.Pp
.Fn CMAC_Final
stores the length of the message authentication code in bytes,
which equals the cipher block size, into
.Pf * Fa out_len .
Unless
.Fa out_mac
is
.Dv NULL ,
it encrypts the last block, padding it if required, and copies the
resulting message authentication code to
.Fa out_mac .
The caller is responsible for providing a buffer of sufficient size.
.Pp
.Fn CMAC_CTX_copy
performs a deep copy of the already initialized
.Fa in_ctx
into
.Fa out_ctx .
.Pp
.Fn CMAC_CTX_cleanup
zeros out both subkeys and all temporary data in
.Fa ctx
and in the embedded
.Vt EVP_CIPHER_CTX
object, frees all allocated memory associated with it,
except for
.Fa ctx
itself, and marks it as uninitialized,
such that it can be reused for subsequent
.Fn CMAC_Init .
.Pp
.Fn CMAC_CTX_free
calls
.Fn CMAC_CTX_cleanup ,
then frees
.Fa ctx
itself.
If
.Fa ctx
is
.Dv NULL ,
no action occurs.
.Sh RETURN VALUES
.Fn CMAC_CTX_new
returns the new context object or
.Dv NULL
in case of failure.
It succeeds unless memory is exhausted.
.Pp
.Fn CMAC_Init ,
.Fn CMAC_Update ,
.Fn CMAC_Final ,
and
.Fn CMAC_CTX_copy
return 1 on success or 0 on failure.
.Fn CMAC_Init
fails if initializing the embedded
.Vt EVP_CIPHER_CTX
object fails.
The others fail if
.Fa in_ctx
is uninitialized.
.Fn CMAC_Update
and
.Fn CMAC_Final
also fail if encrypting a block fails, and
.Fn CMAC_CTX_copy
if copying the embedded
.Vt EVP_CIPHER_CTX
object fails, which can for example happen when memory is exhausted.
.Pp
.Fn CMAC_CTX_get0_cipher_ctx
returns an internal pointer to the
.Vt EVP_CIPHER_CTX
object that is embedded in
.Fa ctx .
.Sh ERRORS
The CMAC code itself does not use the
.In openssl/err.h
framework, so in general, the reasons for failure cannot be found out with
.Xr ERR_get_error 3 .
However, since the
.Xr EVP_Cipher 3
functions are used internally, entries may still get pushed onto
the error stack in some cases of failure.
.Sh SEE ALSO
.Xr EVP_aes_128_cbc 3 ,
.Xr EVP_Cipher 3 ,
.Xr EVP_DigestInit 3 ,
.Xr EVP_PKEY_CTX_new_id 3 ,
.Xr HMAC 3
.Sh STANDARDS
.Rs
.%A Morris Dworkin
.%T "Recommendation for Block Cipher Modes of Operation:\
 The CMAC Mode for Authentication"
.%I National Institute of Standards and Technology
.%R NIST Special Publication 800-38B
.%U https://doi.org/10.6028/NIST.SP.800-38B
.%C Gaithersburg, Maryland
.%D May 2005, updated October 6, 2016
.Re
.Sh HISTORY
These functions first appeared in OpenSSL 1.0.1
and have been available since
.Ox 5.3 .
